An exhaust gas produced by incineration of wastes in an incinerator contains detrimental components such as hydrogen chloride and sulfur oxide. In addition, a fly ash collected from the exhaust gas contains detrimental heavy metals such as zinc, lead and cadmium. It is the conventional practice therefore to add an alkaline substance such as slaked lime, sodium carbonate or calcium carbonate to the above-mentioned exhaust gas discharged from the incinerator while the exhaust gas passes through a flue of the incinerator to remove such detrimental components as hydrogen chloride and sulfur oxide contained in the exhaust gas, then collect a fly ash mixed in the exhaust gas by means of a dust collector, and release the exhaust gas through a chimney to the open air.
The fly ash collected from the exhaust gas as described above has a strong alkalinity under the effect of the alkaline substance added to remove the detrimental components such as hydrogen chloride and sulfur oxide contained in the exhaust gas. For such a fly ash, which is usually dumped to a land site for reclamation, it is necessary prior to dumping to apply a treatment to make insoluble detrimental heavy metals contained in the fly ash.
For the purpose of making insoluble detrimental heavy metals contained in the fly ash, the following methods are known:
(1) A method comprising adding cement and water to the fly ash and kneading the mixture into a lump;
(2) A method comprising adding molten asphalt to the fly ash and kneading the mixture into a lump;
(3) A method comprising placing the fly ash in a tank containing water, neutralizing the mixture by means of an exhaust gas from an incinerator blown into the tank to convert heavy metals in the fly ash into insoluble compounds, then removing water, and forming the insoluble compounds into a lump;
(4) A method comprising adding an acid and a sulfide to the fly ash to convert heavy metals in the fly ash into insoluble compounds, then removing water, and forming the insoluble compounds into a lump; and
(5) A method comprising washing the fly ash by means of water having a weak alkalinity as represented by a pH of about 10, to convert heavy metals in the fly ash into insoluble compounds, then removing water, and forming the insoluble compounds into a lump.
The method (1) above of forming the fly ash into a lump by means of cement and the method (2) above of forming the fly ash into a lump by means of molten asphalt have the following problem: after dumping, heavy metals such as lead, which are easily soluble into a liquid having a high pH value, may be dissolved from the lumpy fly ash.
The method (3) above of treating heavy metals contained in the fly ash through neutralization has the following problem: the steps for neutralizing, dewatering and treating waste water produced in dewatering require complicated facilities, thus leading to a high cost.
The method (4) above of treating heavy metals contained in the fly ash by means of an acid has the following problem: the steps for acid treatment, dewatering and treating waste water produced in dewatering require complicated facilities, consuming much acid, thus resulting in a high cost.
The method (5) above of washing the fly ash by means of weak-alkaline water has the following problem: washing requires water in a large quantity, and the reaction for converting heavy metals contained in the fly ash into hydroxides or carbonates requires a long period of time, leading to huge-scale facilities.
Under such circumstances, there is a strong demand for the development of a method for making insoluble heavy metals contained in a strong-alkaline fly ash, as collected from an exhaust gas discharged from an incinerator, easily and economically and stably for a long period of time, but such a method has not as yet been proposed.